Some mobile terminals are configured to support communications via two or more carrier frequencies. For mobile terminals that do support communications via two or more frequencies, the communications conducted by the mobile terminal may be handed over from a first carrier frequency to a second carrier frequency in certain circumstances, such as in instances in which the first frequency is more heavily loaded or has deteriorated in signal strength to a point that the second frequency is much stronger.
In order to facilitate inter-frequency handovers, a mobile terminal may maintain a virtual active set. A virtual active set is a set of cells having a carrier frequency that is different than the current carrier frequency being utilized by the mobile terminal for communications. By measuring various parameters of the cells that comprise a virtual active set, such as parameters representative of the signal strength of the cells, the mobile terminal may be prepared to support a soft handover immediately after an inter-frequency handover to the carrier frequency supported by the cells of the virtual active set. In this regard, following an inter-frequency handover, the cells of the virtual active set may comprise an initial active set of cells at the new carrier frequency. As such, the mobile terminal may be prepared to effect a soft handover utilizing a cell of the active set so as to facilitate communications at the new carrier frequency.
In order to construct a virtual active set, the network, such as a radio network controller (RNC), may provide configuration information to the mobile terminal to guide the formation of the virtual active set including the maximum number of cells that are to be included within the virtual active set. A virtual active set may then be defined by the mobile terminal based on measurements that the mobile terminal makes of cells with carrier frequencies different than the current carrier frequency. In this regard, the mobile terminal may select the cells that appear to have the strongest signals or are otherwise the best, such as in terms of downlink received signal code power (RSCP) and/or downlink Ec/N0, and/or the cells that have the lowest downlink path loss from among those cells that support communications on the other carrier frequency. Once the virtual active set has been constructed, the mobile terminal may update the virtual active set based on ongoing measurements of the cells operating at the other carrier frequency.
The network, such as the RNC, may also direct a mobile terminal to make inter-frequency measurements by sending a message, such as a measurement control message, to the mobile terminal. The measurement control message may define the manner in which the mobile terminal will report the inter-frequency measurements to the network. For example, a number of predefined events may be known to both the RNC and the mobile terminal and the measurement control message may indicate the occurrence of which event(s) will trigger the mobile terminal to report the inter-frequency measurements. For example, the events may include a change in the best carrier frequency, a combination of the estimated quality of the current carrier frequency being below a predefined threshold and the estimated quality of another carrier frequency being above a predefined threshold, the estimated quality of another carrier frequency being above a predefined threshold, the estimated quality of the current carrier frequency being below a predefined threshold, the estimated quality of another carrier frequency being below a predefined threshold or the estimated quality of the current carrier frequency being above a predefined threshold. The mobile terminal may gather the inter-frequency measurements of the cells operating at the other frequency and may monitor the measurements to determine if the event of interest, as defined by the measurement control message, has occurred. If the event of interest has occurred, the mobile terminal may provide the measurements to the network, such as to the RNC, via a measurement report. Alternatively, if the event of interest has not occurred, the mobile terminal may continue to collect the measurements.
Although useful in supporting inter-frequency handovers between macro cells, the inter-frequency handover process is generally less effective or even ineffective with respect to a mobile terminal that desires to transition from a macro cell to a femto cell, such as a home node B. In this regard, a mobile terminal cannot generally maintain a virtual active set that includes both macro cells and femto cells as the measurements related to the macro cell will be materially different than the measurements for a femto cell.
Recently, efforts have been made to enable the provision of closed subscriber groups (CSGs) to enable restricted access to particular CSG cells, such as particular femto cells, e.g., home node Bs, for particular groups of subscribers. CSGs may be useful for particular organizations or businesses that wish to define a group of users that may be enabled to freely access a base station, node or access point associated with the CSG, but may have restrictions for enabling access to the cell by individuals outside of the group. CSGs may also be useful in connection with individually established networks within private homes. In this regard, for example, a CSG may typically define a group of users (e.g., subscribers) that are enabled to access a particular CSG cell. As such, individuals that are not members of the group may not be able to access the CSG cell. Inbound mobility to a CSG cell on a carrier frequency that is different than the current carrier frequency being utilized by the mobile terminal for communications may be somewhat challenging, however, since measurement evaluations based on a virtual active set that includes one or more CSG cells are not suitable for CSG cells since mobile terminals are not generally configured to affect a soft handover between CSG cells.